This application claims the priority of German Patent Document 001 23 367.5, filed in Germany, Oct. 31, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a device for producing metal diecast parts, particularly made of nonferrous metals, having a hot-chamber diecasting machine with an ascending duct constructed in a casting vessel and having a mouthpiece arranged in front of a gate system, as well as having a gate in front of a diecasting mold, the cross-section of the gate being adapted to the respective molten metal.
Hot-chamber diecasting machines are known which have a pertaining mold construction. During hot-chamber diecasting, the nonferrous metals zinc and magnesium and, to a lesser extent, lead or tin are cast. Metal has the characteristic of cooling rapidly. In order to achieve the best casting quality, diecasting therefore takes place at a high speed and at a high pressure. In this case, the mold filling operation takes between 5 ms and 30 ms (milliseconds) depending on the size of the parts and the minimal wall thickness. The closing force of the hot-chamber machines amounts to up to 10,000 kN.
In the case of the casting operation, certain experimental values exist for calculating the gate system which, for example, with respect to zinc, are at a maximal gate velocity of approximately 50 m per second and, with respect to magnesium, are at maximally 100 m per second. At the used high melting temperatures of approximately 650xc2x0 C. in the case of magnesium and approximately 420xc2x0 C. in the case of zinc, these nonferrous metals in the liquid condition are almost as liquid as water. In order not to exceed the above-mentioned gate velocity, the cross-section of the gate surface, that is, the portion of the gate system which afterwards permits the separation of the gate part from the mold, must have a correspondingly designed cross-section.
It is also known (xe2x80x9cThe Operation of the Diecasting Machinexe2x80x9d, Society of Die Casting Engineers, Detroit/USA, Copyright 1972, Page 7) that, in the hot-chamber diecasting method, a fan or a tangential gate is used in order to be able to uniformly fill the diecast part. Particularly when multiple molds are used, this leads to a complex gate system which, after the cooling of the metal, remains as a residue which cannot be used. This gate fraction, relative to the diecast part, has a weight fraction of between 40% and 100%. The gate fraction which remains after each shot is subsequently melted again which, however, requires considerable additional energy expenditures. In addition, material is lost because of melting loss, deburring of the gate system and its recycling.
It is an object of the present invention to provide, in the case of a device of the initially mentioned type, a further development in which significantly less gating fraction can be used.
For achieving this object, the invention, in the case of a device of the initially mentioned type, provides that the gate is part of a hot-duct gating system which provides a heating of the ducts and of the nozzles leading to the mold.
By means of this further development, it becomes possible to keep the material in the liquid condition in the always required partially very complex gate ducts, so that, after the cooling of the metal in the mold, no cooling occurs of the material situated in the gate ducts. This material can be used again during the next shot.
In the case of injection molding machines for plastic materials, it is basically known to use hot-duct systems. However, since the heat-conducting characteristics of plastic differ decisively from those of metals, an application of the design of such hot-duct systems, in the case of which the mold can be filled in a punctiform manner or by way of a tunnel, is not possible.
As a further development of preferred embodiments of the invention, it is provided that nozzle tips are fitted to the nozzles which are provided with a comb-type gate system or a fan-type gate system and directly adjoin the contour of the part, in which case the comb-type gate system or the fan-type gate system forms the gate or is disposed directly in front of the latter. This further development has the advantage that the molten metal situated in the gate cross-section of the nozzle tips, after the filling of the mold, changes at least into the semisolid condition, because the nozzle tips themselves are not heated. As a result this material prevents that, after the opening of mold, metal flows in out of the hot-duct system or flows through the latter back into the mouthpiece, the ascending duct or the casting vessel.
As a further development of preferred embodiments of the invention, the nozzle tips and the nozzles are in each case provided with conical plug connections which, also at the above-mentioned very high temperatures of from 650xc2x0 C. and 420xc2x0, ensure a sufficient sealing-off by the placing of metal on metal.
In this case, the nozzle tips themselves can be fitted to heated nozzles and the nozzles, in turn, can be fitted to heated ducts.
As a further development of preferred embodiments of the invention, the nozzle tips can be constructed to be adapted to the respectively used mold of the part to be produced. In this case, the nozzle tips can be fitted laterally or centrally onto this mold.
An alternative for preventing the return flow of the liquid metal into the ascending line and the casting vessel can be achieved, according to certain preferred embodiments of the invention, in that a nozzle tip is assigned to the mouthpiece, which nozzle tip rests against the gate system, is unheated and in which a plug is formed after the filling of the mold, which plug, in turn, can prevent the return flow of the molten mass situated in the mouthpiece and the ascending tube back to the casting vessel. During the next shot, this plug is pressed into the hot-duct system, where a corresponding receiving space for the plug is provided in which the plug arrives and will thereby not further hinder the continued injection of liquid material. The plug will melt again in the hot-duct system.
In order to avoid a return flow into the casting vessel in every case, additionally to or instead of the above-mentioned alternative with a mouthpiece, it may also be provided, according to certain preferred embodiments of the invention, that a return valve is arranged in the ascending duct. A return valve may also be arranged in the casting plunger, so that the disadvantage which had previously occurred in the case of diecasting machines, which is when, during the withdrawal of the casting plunger from the ascending duct, there is no afterflow of material, as a result of the vacuum occurring in the casting cylinder, material flows past the plunger rings into the casting cylinder, can be avoided. As a result of the arrangement of a return valve in the casting plunger, material can now flow directly from the casting vessel through the casting plunger into the casting cylinder. The return valves which are to be used in this case should be comprised of a highly heat-resistant material or of ceramics in view of the occurring high temperatures.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.